This invention relates to unsaturated polyester resin compositions and sheet-like and bulk-like molding compounds obtained by impregnating glass fibers with such resin compositions, and molded products obtained therefrom which have superior paintability and hence are useful for coating automotive exterior body panels and other exterior parts.
Fiber-reinforced plastics (FRP) using unsaturated polyester resins have superior mechanical strength and resistance against heat, water and chemicals and hence have been extensively used, for example, in bathtubs, water tank panels and bathroom sinks. Since sheet molding compounds (SMC) and bulk molding compounds (BMC) using unsaturated polyester resins are easy to mold and have high production efficiency, they have been commonly used as FRP molding materials. In response to the increased demand for light-weight SMC and BMC, many attempts have been made recently to reduce the density of SMC and BMC. Although the most frequently used methods are by the addition of hollow glass microsphere to a resin composition, the water-resistance is significantly affected by the addition of such hollow glass microsphere in the case of automotive exterior body panels and other exterior parts (such as engine hoods, roofs, trunk lids, spoilers, air intakes and rocker panels), which are usually coated. If a molded product, thus coated, is soaked in warm water, for example, defective adhesion of the coating and blisters will occur, rendering the product unusable as a practical molded product. Moreover, although metallic oxides are frequently used with SMC and BMC as thickeners for resins in order to improve their workability, the addition of hollow glass microspheres has a significant inhibitive effect on the increase in viscosity, and it has been difficult to obtain a molding material which can be easily handled, or worked with. It is for this reason that there have been strong demands for the development of molding materials which are light in weight and have superior paintability and workability. Although a BMC containing hollow glass microspheres having an effective density of 0.3-0.4 g/cm.sup.3 and treated with acid at least once, has been proposed (Japanese Patent Publication Tokkai 1-234434) as such a molding material, paintability and workability are not sufficiently improved even if this material is used. The inventors have accomplished the present invention as a result of their diligent studies into molding materials having all of these characteristics.